Tracking Earth Changes with Satellite Images (Forwarded)

Media Relations
Caltech
Pasadena, California

Contact:
Elisabeth Nadin, (626) 395-3631

December 14, 2007

Tracking Earth Changes with Satellite Images

SAN FRANCISCO, Calif. -- For the past two decades, radar images from
satellites have dominated the field of geophysical monitoring for natural
hazards like earthquakes, volcanoes, or landslides. These images reveal
small perturbations precisely, but large changes from events like big
earthquake ruptures or fast-moving glaciers remained difficult to assess
from afar, until now.

Sebastien Leprince, a graduate student in electrical engineering at the
California Institute of Technology, working under the supervision of geology
professor and director of Caltech's Tectonics Observatory (TO),
Jean-Philippe Avouac, wrote software that correlates any two optical images
taken by satellite. It has proved extremely reliable in tracking large-scale
changes on Earth's surface, like earthquake ruptures, the mechanics of
"slow" landslides, or defining the fastest-moving sections of glaciers that,
due to global warming, have recently increased their pace.

Leprince will describe his software and results of many of its applications
on December 14 at the annual meeting of the American Geophysical Union (AGU)
in San Francisco. His research will also be featured in the January 1 issue
of Eos, AGU's weekly newspaper.

When the technique called InSAR, which uses radar images to reveal details
about ground displacement, was introduced, it was quickly embraced. No
longer did geoscientists have to rely solely on measurements made by troupes
of field geologists or by ground-based devices that might not have been
optimally placed. But, says Leprince, "InSAR is physically limited: it's
good for small displacements but not for large ones. The radar resolution
isn't enough to look at deformation with a large gradient."

Using optical images to complement the radar-based InSAR technique seemed
like a natural step. When Leprince began grappling with the idea in 2003, he
found several baby steps had been taken. "Satellite image correlation was
not a science yet, it was more like an art," he says. The first attempts,
reported in 1991, were inconclusive but promising. Since then, several teams
of scientists had worked on the problem independently. Some had even
developed it well enough to monitor glacier flow.

The major obstacle Leprince faced in developing optical image correlation
software was that there were several steps involved but no one knew in which
order to take them. "Errors came from everywhere, but where exactly?" he
noted. "And we found at least one major flaw in each step."

Three of the four main steps involve correcting geometric distortions innate
to taking pictures from space and projecting them onto a surface. The first
step matches coordinates of the satellite image with coordinates on the
ground. "This is not new, but the approximations being made were not okay,"
says Leprince. The second step describes the satellite's position in its
orbit at the time it took the photo. This is just like in everyday life --
you need to know how your camera was oriented when you show off a photo you
snapped. In the next step, which Leprince says people never knew they were
doing wrong, the image is correctly wrapped onto topography. Finally, the
images are precisely combined -- or coregistered -- in order to measure
surface displacements accurately.

"What is important is that we identified the steps and took each one
independently and did an error analysis for each step to see how errors
propagated," says Leprince. His program, which he calls COSI-Corr and which
was packaged by the TO's software engineer Francois Ayoub for official
release this year, takes all of these steps automatically in just a few
hours of processing time. "You start the program, you go home, you have a
nice weekend on the beach, and it's done."

The paper describing the software Leprince developed appeared in the June
2007 issue of the journal IEEE Transactions on Geoscience and Remote
Sensing. COSI-Corr can now combine any images taken by different satellite
imagers from different incidence views. For example, to analyze displacement
from the 1999 Hector Mine earthquake near Twenty-Nine Palms in California,
Leprince correlated a SPOT 4 image with an ASTER image. This had never been
done before. It takes only a few hours to process.

Using his technique, Leprince has precisely measured offset from several
notable recent earthquakes, including 2005 Kashmir, Pakistan; 2002 Denali,
Alaska; 1999 Hector Mine and Chi Chi, Taiwan; and 1992 Landers, California.
In the case of earthquakes, the image correlation technique can be used to
map in detail all fault ruptures and to measure displacements both along and
across the fault. Uncertainties, typically within centimeters for
10-15-meter-resolution images, are extremely low.

The day after Leprince released his software through the TO website, he was
contacted by a geologist in Canada asking how the technique could be used to
study glacier flow. Radar images cannot analyze glaciers because they move
too fast and ice melting poses a problem. "The tectonic application was
pretty well set up and we'd tested it thoroughly," says Leprince. "So we
extended it to glaciology." And then to other studies as well.

What's tricky about studying glacier flow is that not only has their pace
picked up in recent years due to climate change, but glaciers have a natural
yearly cycle of ice gain and loss. The two signals can be discerned with
cross-correlation of optical imagery. Leprince's method was used to study
Mer de Glace glacier in the Alps, which flows at around 90 meters per year.
The optical images provide a full view of the ice flow field, pinpointing
exactly where the glacier is moving fastest. The same approach was taken
with a landslide above the Alpine town of Barcelonnette in eastern France.
Benchmarks had been planted to monitor the landslide's flow, and Leprince's
correlation methods showed that all 38 of them missed the fastest-moving
region. While the landslide is moving slow now, the town will be threatened
when the landslide detaches and descends rapidly.

There are many more applications for correlating optical images to monitor
Earth surface changes. Caltech geologists and their collaborators began to
apply it to studying dunes, which radars cannot image, after they were
contacted by labs in Egypt who need information on dune migration for urban
planning.

"Radar interferometry is a huge technique, but you can only measure half of
the world with it. Now we can measure the other half with this technique,"
comments Leprince. "The biggest thing is what's to come."

COSI-Corr and many of its applications will be presented by Leprince on
Friday morning, December 14, in Moscone South Exhibit Hall B. To learn more
about the technique, visit
http://www.tectonics.caltech.edu/sli...eis/index.html